This application is based on Japanese Patent Application Nos. Hei 9-264102, Hei 9-267037 and Hei 10-217039, respectively filed in Japan Sep. 29, 1997, Sep. 30, 1997 and Jul. 31, 1998, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an organic electroluminescent element.
2. Description of the Prior Art
The organic electroluminescent element emit light in accordance with electric signals and is constituted of an organic compound as a luminous material.
The organic electroluminescent element is basically constituted of an organic luminous layer and a pair of electrodes with the luminous layer sandwiched between the electrodes. Luminescence is a phenomenon in which electrons are injected from one electrode and holes are injected from the other electrode so that the luminous material in the luminous layer is excited to a higher energy level and then the excited luminous material goes down to a ground state to emit extra energy as light.
In addition to the above basic structure, a hole-injection layer for injecting holes is formed on the hole-injection electrode or an electron-injection layer is formed on the electron-injection electrode in order to improve luminous efficiency.
U.S. Pat. No. 3,530,325 discloses an example of electroluminescent elements in which single crystal anthracene is contained as a luminous material. Japanese Patent Laid-Open No. Sho 59-194,393 proposes the combination of a hole-injection layer with an organic luminous layer. Japanese Patent Laid-Open No. Sho 63-295,695 proposes the combination of an organic hole-injection transporting layer with an organic electron-injection transporting layer.
Those laminated eletroluminescent element has a structure in which an organic fluorescent material, a charge-transporting organic compound (charge-transporting material) and electrodes are laminated. Holes and electrons injected from respective electrode move in the charge-transporting material and are recombined to emit light. An organic pigments which can emit fluorescenct light, such as 8-qunolinol aluminum complex and cumarin, are used as the organic fluorescent material. The electron-transporting material is, for example, exemplified by amino compounds, such as N,Nxe2x80x2-di(m-tolyl)N,Nxe2x80x2-diphenyl benzidine and 1,1-bis[N,N-di(p-tolyl)aminophenyl]cyclohexane, and 4-(N,N-diphenyl)aminobenzaldehyde-N,N-diphenyl hydrazone. In addition, porphyrin, such as cupper phthalocyanine, is proposed.
By the way, organic electroluminescent element has a high luminous properties. Stability at light-emitting time and preserving stability, however, are not sufficient for the electroluminescent element to be put into practical use. The stability of charge-transporting material is pointed out as one of problems with respect to the light-emitting stability and preserving stability of the element. A layer constituted by an organic material in the electroluminescent element is several tens of nm to several hundred of nm in thickness, being very thin. A voltage applied to the layer in unit of thickness is very high. As heat is emitted at light-emitting time or in electriferous conditions, the charge-transporting material is required to have electrical stability, thermal stability or chemical stability.
Japanese Patent Laid-Open Nos. Hei 2-15,595, Hei 3-37,994, Hei 4-132,191 and Hei 5-121,172 disclose that materials other than aluminum is used as cathode in order to lower light luminescence-starting voltage of an electroluminescence element.
Japanese Patent Laid-Open Nos. Hei 4-132,189 and Hei 7-268, 317 disclose that a layer constituted of a mixture of an electron transporting material and metal is used as an electron-injection layer.
The handling of the material other than aluminum is difficult in layer-forming conditions and the material is liable to be oxidized. The same problem arises when the mixture of the electron transporting material and metal.
When metal used for formation of an electron-injection layer is oxidized at vapor deposition time, powdery ones are formed. A uniform layer can not be formed. White haze occurs. Metallic layer is not formed. The formed one can not be used as an electrode.
A layer having excellent properties as an electron-injection layer has not been obtained yet as things are.
The present invention is to provide an electroluminescent element having high luminous strength and displaying stable performance even if used repeatedly.
The present invention relates to an organic electroluminescent element comprising an anode, an organic luminous layer, an electron-injection layer and a cathode, in which the electron-injection layer comprises a metal oxide or a metal halide, and a different material therefrom.